The invention relates to a C-W radar responder, which receives a frequency and amplitude-modulated carrier whose average frequency is equal to some gigahertz. The responder comprises means for sampling the received carrier at a frequency which is at least equal to double the amplitude-modulation frequency, which is located between some dozens and some hundreds of kilohertz. The responder also comprises an aerial, an amplifier, a delay line having a time delay, at least a first switch having two positions, which acts as the sampling device, a clock generator, an input and an output.
Responders or beacons, beacons being defined as ground-based responders, are usually used to amplify an echo detected by a radar system and for the identification of aircraft detected by the radar system. Such responders may, for example, in conjunction with distance measuring equipment of the radio altimeter type on board helicopters be of assistance during flying in formation. When the responder is a beacon and a radio altimeter is provided on board the helicopter, the responder functions to help, for example, during landing or during navigation of the helicopter with a range up to some dozens of kilometers. The identification of the responder may, for example, be done by means of an amplitude modulator or a single-sideband modulator associated with the responder.
When a responder is used in relation with a C-W ("continuous-wave") radar transmitter, for example a radio altimeter of the type described in French Patent specification No. 1,557,670, (corresponding to U.S. Pat. No. 3,588,899) which can transmit a carrier with a frequency of several gigahertz, it is necessary to maintain a certain phase relationship between the received carrier and the re-transmitted carrier, so that in the radar transmitter the phases of the signals transmitted by the radar and the signals which are re-transmitted by the responder can be compared.
For the case of a C-W radar responder it is not necessary to use an amplifier directly. It is possible to change to a lower intermediate frequency, to amplify at that lower intermediate frequency and to re-transmit with a frequency which may be the frequency of the carrier received by the responder, or a different frequency. Whatever the case, the carriers received by the responder and re-transmitted by it are then of the same nature, that is to say all components of the signal are retained. With the very high frequency amplifiers of present day technology the change to a lower intermediate frequency can be avoided, and direct amplification is possible. The invention relates more particularly to this type of amplification, which will be included by way of non-limitative example in the description given hereinafter.
For the preferred uses, described within the scope of the invention and in order to have a basis for comparison with the prior art, the powers to be re-transmitted are, for example, on the order of some milliwatts and correspond to a minimum gain between the received and the re-transmitted carriers, which may be over 80 dB. In addition, in certain applications, an address which may be the responder address, is inserted by a modulator and superimposed on the re-transmitted signal, for example, in the form of a frequency shift of a predetermined value.
The simplest circuit diagram of the known C-W responder comprises an amplifier, whose input is connected to a receiving aerial and the output to a transmitting aerial. Such an arrangement is not suitable for the considered uses, as the unavoidable couplings between the two aerials would make the system unstable, unless one is satisfied with a very low gain to the order of, for example, 30 dB. The necessary decoupling between the two aerials must exceed 80 dB for the applications considered here. Such decoupling is difficult to realize, both for a ground-based beacon, because of the parasitic carriers reflected from the soil, (Larsen effect), and for a responder installed on board an aircraft, because it is difficult to space the two aerials sufficiently apart.
It may be desirable to have only one aerial instead of two, for example in the case where it is necessary to realize a difficult, accurate angular adjustment, or in the case where only little space is available on board an aircraft. In that case, one could use in known manner one receiving signal aerial and a circulator, which connects the aerial to the input and to the output of the amplifier. In such a responder, the decoupling problems are still more critical than in the case described in the preceding paragraph, as it is practically impossible to avoid a reflection which has a certain standing wave ratio, and it is difficult to get a standing wave ratio below 1.2, which corresponds to only 1% of the reflected energy, that is to say a decoupling less than 20 dB. It would therefore be necessary to use an amplifier having a gain factor less than 20 dB, which is insufficient.
In another prior art type of a responder, a portion of the received carrier is amplified, stored in a delay-line and re-transmitted with a predetermined frequency, after having been amplified for the second time. Such a responder comprises the cascade arrangement of a first amplifier, whose first stage is also used to filter the signal, a delay line, a modulator which determines the predetermined frequency, and a second amplifier. By means of a two-position switch a first terminal of which is connected to a transmit-receive aerial, the aerial can be connected to the input or to the output of the responder, either in the receive position or in the position for the re-transmission. The combined use of a delay line and a two-position switch effects an efficient decoupling between the received signal and the re-transmitted signal, because of their displacement in time, and enables an amplification between these two signals which can be more than 80 dB. This advantage is the result of the fact that the received wave is sampled by the two-position switch, which acts as a sampling device, as a clock generator consisting of a control circuit connected to an oscillator controls the change-over of the switch from the one to the other position at a constant predetermined rate Fd. Such sampling, is not a serious drawback, provided that the relationship between the received signal and the re-transmitted signal can be maintained in accordance with the sampling theorem, alteratively called the Shannon formula. This is accomplished if sampling is done at a rate Fd, which is at least equal to double the maximum amplitude modulation frequency of the signal received by the responder. When this requirement is satisfied, which, as explained above, is rather easy, such a responder is denoted a pseudo C-W responder. In the category of C-W responders the invention relates more particularly to this type of pseudo C-W responder in which a clock generator determines both the sampling frequency and the cycle ratio, that is to say the ratio between the receiving period and the total period Td, which is equal to the inverse of the sampling rate Fd. The clock generator is independent, that it is say, it does not require any synchronisation with respect to any other signal. A pseudo C-W responder of this art is described in, for example, French patent specification no. 2,343,258, (corresponding to U.S. Pat. No. 4,151,552) with reference to the FIGS. 9 and 10 of that specification.